This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to JP 11-166082 filed in Japan on Jun. 11, 1999, the entire content of which is herein incorporated by reference.
1. Technical Field
The present invention relates to a method for continuous casting of a steel such as a peritectic steel at high speed. The method enables a steady operation due to a prevention of a break-out and an periodic fluctuation of molten steel level during the casting, and can produce a slab having excellent surface quality; i.e., a slab having no longitudinal cracks on the surface.
2. Background Art
In a method for continuous casting of a steel slab, in view of slab quality and productivity, generally, a slab with a thickness of 150-300 mm is cast at a speed of about 1-2 m/minute. In recent years, in consideration of reduction in construction cost of related equipment and the number of operators, casting of a slab with a thickness and shape similar to those of a product has been attempted. Particularly, in the production of hot coils, combination of a continuous casting method for a thin slab and a rolling method carried out by means of a simple hot strip mill arranged downstream on a casting line is in practical use. In such a simple hot strip mill, generally, a thin slab with a thickness of 40-80 mm is used as a material to be rolled.
It is difficult to practice a technique for casting a thin slab with a thickness of 40-80 mm by means of a generally used mold in which the inlet and outlet are of the same thickness. The thickness of a material used in a submerged entry nozzle cannot be increased, and the nozzle is susceptible to melting loss. Thus, in the course of casting, an accident in which the nozzle breaks and casting cannot be carried out may occur.
In order to solve such a problem, there is a method for casting a thin slab, employing a mold having an outlet thickness of 40-80 mm and an inlet thickness which is greater than the outlet thickness at a position at which a submerged entry nozzle is inserted. In another method for casting a thin slab, a thin slab with a thickness of 80 mm to 120 mm is cast by means of a mold in which the inlet and outlet are of the same thickness, and the slab containing a liquid core is subjected to reduction in a continuous casting apparatus, to thereby obtain a thin slab with a thickness of 40-80 mm. In either method, the thickness of a submerged entry nozzle can be increased, and breakage of the nozzle due to melting loss thereof rarely occurs. Hereinafter, a method of continuous casting of the above-described thin slab will be described generally as a continuous casting methods for obtaining a thin slab with a thickness of 40-120 mm.
In a simple hot strip mill arranged on a casting line, which follows continuous casting of thin slabs, productivity is as high as approximately 200-400 ton/hour, and thus two continuous casting apparatuses may be installed to one hot strip mill. However, in order to facilitate the operation of both the continuous casting apparatus and the strip mill, generally, one continuous casting apparatus is arranged. When only one continuous casting apparatus is employed, casting must be carried out at a speed of at least 3-5 m/minute in order to maintain productivity of the hot strip mill.
However, when casting speed increases, the amount of molten slag which flows into a gap between the inner wall of a mold and a solidified shell decreases. Here, a molten slag is formed from a mold powder which is added to the surface of molten steel in a mold and melted. When the inflow amount of molten slag decreases and the thickness of molten slag decreases, a solidified shell tends to bind to the inner wall of a mold, due to insufficient lubrication. Therefore, in an extreme case, break-out may occur. In order to maintain the inflow amount of molten slag, mold powder with a lower solidification temperature and viscosity is employed. However, when mold powder with a lower solidification temperature and viscosity is employed, the thickness of molten slag tends to be uneven. Thus, a solidified shell in a mold is not cooled evenly, and longitudinal cracks tend to form on the surface of a slab.
Incidentally, it is well known that a molten steel of a peritectic steel is solidified unevenly, and thus longitudinal cracks tend to form on the surface of a peritectic steel slab.
As described above, when peritectic steel is cast at a speed of at least 3-5 m/minute to thereby obtain a thin slab with a thickness of 40-120 mm, longitudinal cracks form in a considerable amount on the surface of the slab due to synergistic effects of uneven solidification and high-speed casting. In addition, break-out tends to occur because of insufficient lubrication.
In order to prevent formation of longitudinal cracks on the surface of a slab in the case in which the slab is cast at high speed, the following methods are proposed. Japanese Patent Application Laid-Open (kokai) No. 193248/1991 discloses a method in which oxides of elements belonging to Groups IIIA and IV, such as ZrO2, TiO2, Sc2O3, and Y2O3, are added to mold powder as crystallization accelerators. In the method, molten slag is crystallized when cooled from a molten state. A solidified shell in a mold is cooled gradually due to crystallization of the slag. When the solidified shell is cooled gradually, the cooling rate of the shell becomes even, and thus formation of longitudinal cracks on the surface of a slab can be prevented. In addition, in the method, the viscosity of molten slag is 1 poise or less at 1,300xc2x0 C., and high-speed casting can be carried out.
Meanwhile, Japanese Patent Application Laid-Open (kokai) No. 15955/1993 discloses a method employing mold powder of low viscosity and high total CaO/SiO2, the ratio of total CaO (mass %) to SiO2 (mass %). In the method, total CaO refers to the sum of CaO contained in mold powder and CaO reduced from the amount of Ca which is assumed to be present as CaF2. When total CaO/SiO2 is as high as 1.2-1.3, molten slag is crystallized when cooled from a molten state. As described above, formation of longitudinal cracks on the surface of a slab can be prevented, due to crystallization of the slag.
However, even when the above methods disclosed in Japanese Patent Application Laid-Open (kokai) Nos. 193248/1991 and 15955/1993 are employed for casting peritectic steel at a speed of at least 3-5 m/minute to thereby obtain a thin slab with a thickness of 40-120 mm, in practice, formation of longitudinal cracks on the surface of the slab and break-out tend to occur. In addition, periodic fluctuation of molten steel level in the vertical direction may occur. In an extreme case, molten steel comes out from the inlet of a mold, and operation cannot be continued. Practically, such a problem has not been solved yet until now.
In view of the foregoing, an object of the present invention is to provide a method of continuous casting of a steel, which method enables a steady operation due to preventing an occurrence of a break-out and an periodic fluctuation in molten steel level in the course of continuous casting of a steel such as a peritectic steel at a high speed of 2.5-10 m/minute, and can produce a slab having no longitudinal cracks on the surface.
The continuous casting method of the present invention is a method for casting a steel such as a peritectic steel at a high speed of 2.5-10 m/minute, in which the steel is cast under the conditions that chemical composition and physical properties of mold powder, mold oscillation, and secondary cooling condition are controlled in a particular range. Mold powder employed in the present invention has a viscosity of 0.5-1.5 poise at 1,300xc2x0 C., and a solidification temperature of 1,190-1,270xc2x0 C. In the mold powder, the ratio of CaO (mass %) to SiO2 (mass %), CaO/SiO2, is 1.2-1.9. A mold oscillation stroke is 4-15 mm, and a specific cooling intensity in secondary cooling of a slab is 1.0-5.0 liter/kg-steel.
In the continuous casting method of the present invention, a mean flow rate of molten steel in a horizontal direction is 20-50 cm/second in the meniscus of molten steel at a position which is located at a distance of xc2xc width of the cavity of the mold from the inside wall of the mold in a width direction, and at a distance of xc2xd thickness of the cavity of the mold from the inside wall of the mold in a thickness direction. The maximum flow rate is preferably 120 cm/second or less in the meniscus of molten steel at the same position mentioned above. Under these conditions, formation of longitudinal cracks on the surface of a slab can be effectively prevented.
In the continuous casting method of the present invention, a slab containing a liquid core is preferably subjected to reduction before completion of solidification. Thus, a slab with a thickness of 40-80 mm can be obtained from a thin slab with a thickness of from more than 80 mm to 120 mm.
Furthermore, in the continuous casting method of the present invention, the ratio of CaO (mass %) to SiO2 (mass %), CaO/SiO2, in mold powder, is preferably 1.2-1.9. Under the conditions, formation of longitudinal cracks on the surface of a slab can be effectively prevented. In addition, lubrication between the inner wall of a mold and a solidified shell is enhanced, and thus occurrence of break-out can be effectively prevented.
The method of continuous casting of a steel of the present invention is preferably applicable to cast, in particular, a steel containing C in an amount of 0.065-0.18 mass %. Steel containing C in the above amount is so-called peritectic steel. As described above, when peritectic steel is cast, longitudinal cracks tend to form on the surface of a slab and periodic fluctuation of molten steel level may occur. The continuous casting method of the present invention is very effective in solving such problems.